Are there other planets like Earth out there? Could they be places that future generations could eventually live? Is our Solar System somehow special? Current research on exoplanets – planets orbiting stars other than our own – aims to answer these questions. We know of hundreds of exoplanets, but one particular class of planet has continued to elude us: a planet the same size as Earth, orbiting a star like our Sun, and with a surface temperature just right forliquid water. Such a planet could be habitable to life as we know it here on Earth.
Advancements in exoplanet astronomy have come from every part of the globe, from dozens of telescopes and from thousands of people. This article will focus on the contributions from the most ambitious program: the Kepler Mission. The mission uses a 1.4 meter telescope in space called Kepler to look for new exoplanets.
One of the primary goals of the Kepler Mission is to determine how many Earth-sized planets are orbiting Sun-like stars with periods of about 1 year. The Kepler telescope looks for exoplanets using the transit method. When a planet passes between its host star and us, or “transits,” it blocks out a small portion of the star’s light proportional to its size (about 1% for a Jupiter-sized planet orbiting a Sun-like star) and causes a measurable dip in the brightness of the host star.
The image below illustrates an idealized exoplanet transit. As a large planet (blue circle) passes between us and its host star(orange circle), the amount of light received by a telescopedrops. First, the planet does not block the star at all and we receive all the light emitted by the star (the planet itself emits very little light). When the planet transits, it blocks a bit of the star from view and the amount of light we receive decreases. Then, as the planet completes its transit, the amount of light received jumps back up to the normal level.
Figure 1. Diagram of an exoplanet transit — As a planet (small blue circle) transits, or passes in front of, its host star (large orange circle), it blocks some of the light.
How rare are transits?
The transit of an Earth-like planet will occur only once a year (since that’s how long it will take for the planet to make one orbit around its star) while the transit itself would last about 12 hours. Therefore, we want to watch each star continuously because we don’t want to miss the transit! The solution to this problem is to put the Kepler telescope into space so that observation doesn’t have to stop just because it’s daytime.
A planetary system must also be viewed from the edge, or else a planet would never pass between us and the star. Only a small fraction of planetary systems out there will be properly aligned with the telescope such that transits are seen at all. For example, for an Earth-sized planet orbiting a Sun-like star, the chance of proper alignment is only 0.5%! This means that Kepler needs to be looking at a lot of stars – in fact, it’s monitoring more than 100,000 .
With more transits, it becomes easier to be confident that you are in fact seeing the signature of a transiting exoplanet. The Kepler scientists consider three transits necessary to be confident that the fluctuations in the received light really are due to a transiting exoplanet. The mission was therefore slated to last for 3.5 years – long enough to observe three or four transits of a planet on a one year orbit.
How difficult are transits to detect?
The stars and planets themselves complicate matters. In the idealized transit diagrammed previously, the light given off by the star is constant. In reality, the brightness of a star will fluctuate, making it more difficult to pick out a transit. Furthermore, planets as small as the Earth result in only a very small decrease in the light received: the brightness drops by only 0.01% during transit! Generally, the more variable a star and the smaller the planet, the more difficult it will be to pick out a transit.
Figure 2. The transit of a 1.4 Earth-radius exoplanet, Kepler-10b  — Many transits have been stacked together to make the transit easier to detect. The phase is the time in hours since mid-transit.
The analysis of Kepler’s ability to detect Earth-like planets around the stars it observes was originally based on the variability of the Sun. Unfortunately, the assumption that our own Sun is a good representative of all stars turned out to be wrong. Most stars are more variable than the Sun so it is more difficult to detect Earth-like planets than initially believed! This could make the difference between discovery of an Earth-like planet within Kepler’s 3.5 year mission and not making that detection. In order to complete its mission goals, the Kepler mission likely needs to be extended to 6 years .
Discoveries from Kepler
In February 2011, Kepler released its first batch of candidate exoplanets. The total number of candidates? 1,235. In the video at <http://www.youtube.com/watch?v=qRJ30fkyiU4>, Dan Fabrycky shows us a view of the Kepler candidates.
Most of these planets haven’t been studied thoroughly enough for the Kepler scientists to confirm that they are indeed planets. To date, there are 24 confirmed exoplanet discoveries from Kepler. I’ll highlight several of the discoveries I find most interesting, but the properties of all these planets can be explored on the Kepler website <http://kepler.nasa.gov/Mission/discoveries/>, news sites, and blogs (e.g. Astrobites <www.astrobites.com>).
THE KEPLER-10 SYSTEM [2,4]
Kepler-10b was the first rocky planet found by Kepler and remains the smallest exoplanet discovered, with a mass at 4.6 times the Earth’s and a radius 1.4 times the Earth’s. That means the average density of this weighty planet is 1.6 times the mean density of Earth.
Figure 3. Artist’s conception of Kepler-10. Because it is so close to its host star, this planet is expected to be very hot.
But don’t send any spaceships yet: with an orbital period of only 0.84 days, it’s also far from habitable. It’s twenty times closer to its star than Mercury is to the Sun and is certainly boiling hot. There’s also a second planet orbiting the star Kepler-10. Kepler-10c is slightly larger and has an orbital period of 45 days, but even though it’s farther away, it would still be too hot for us.
THE KEPLER-11 SYSTEM 
The host star in this system has not just one or two planets orbiting it, but six! The smallest of the Kepler-11 planets has a diameter 2.6 times Earth’s and has a mass 2.3 times Earth’s, while the largest come in at about the size of Uranus. What’s truly remarkable about this system is that the entire system could fit entirely within the orbit of Mercury: the planet orbiting furthest out in the Kepler-11 system is at about the same distance as the planet closest to the Sun in our Solar System.
Figure 4. Artist’s conception of the Kepler-11 system. This planetary system includes six planets.
In Star Wars, Luke’s home planet Tatooine orbits a binary star, so there are two suns in the sky. In September, Kepler announced the discovery of its very own Tatooine: a planet orbiting a pair of stars. Unlike Tatooine, Kepler-16b is a gas planet and an unlikely place to find Jedi. But, you would still get some spectacular sunsets!
Figure 5. Artist’s conception of the Kepler-16 system, in which a planet orbits two stars.
When astronomers first look at this system, it was immediately obvious that it was in fact a binary star: there were large, noticeable drops in received light when one of the stars passed in front of the other. But then they noticed other dips – those caused by an exoplanet that transits both stars.
What comes next?
The story isn’t over once an exoplanet has been found. From the transit method, only the exoplanet’s size and the period of its orbit are known. By applying other methods, its mass can also be determined; with mass and size, its average density is also known. The planet’s density hints at its composition and helps astronomers to understand what it is made of: is it a rocky planet, like Earth, or is it more similar to Jupiter or Neptune? Other avenues of research being pursued involve trying to understand whether a planet has a moon, what elements are present in its atmosphere, and how it might have formed and evolved.
Elisabeth Newton is a Harvard graduate student in astronomy studying low-mass stars. She writes for astrobites, a daily astronomy blog.
 Kepler Mission Website, http://kepler.nasa.gov/. Accessed 10/15/2011.
 Batalha, N.M. et al. The Astrophysical Journal, Vol. 729. 2011. http://iopscience.iop.org/0004-637X/729/1/27/
 Batalha, N.M. et al. Oral presentation at Extreme Solar Systems II, Jackson Hole, WY. 2011.
 Fressin, F. et al. Kepler-10 c: a 2.2 Earth Radius Transiting Planet in a Multiple System. The Astrophysical Journal Supplement Vol. 197. 2011.
 Lissauer, J.J. et al. A closely packed system of low-mass, low-density planets transiting Kepler-11. Nature Vol. 470. 2011.
 Doyle, R.L et al. Kepler-16: a transiting circumbinary planet. Science Vol. 333. 2011.